In restorative dentistry, implantable dental devices are used to achieve reinforcement of damaged or diseased teeth and permanent replacement of missing teeth that may have been lost or need to be replaced due to trauma, decay or disease. Dental implants can be implanted into the root canal space of a tooth to reinforce existing tooth structure and can be implanted into the root canal space of a tooth or a cavity within alveolar bone to provide means for fixation of a dental prosthesis such as a crown or bridge.
An implantable dental fixation device typically comprises an artificial root structure which is implanted into alveolar bone of the jaw or root canal space of a tooth and an abutment to allow for attachment of a dental prosthesis such as a crown or bridge.
The majority of known intraosseous dental fixation devices are cylindrical or screw-shaped devices formed from titanium or a titanium-based alloy, which can be inserted into a pre-drilled hole in the alveolar bone of the jaw. Such devices are generally designed with the aim of achieving stable long-term fixation within alveolar bone, preferably enhanced by osseointegration. Osseointegration is a process in which a direct structural and functional connection is formed between living bone and the surface of an artificial implant. Osseointegration allows implant integration with surrounding bone by means of osteoblasts growing on the surface of the device. However, osseointegration takes time and cannot be relied upon as the sole means for implant fixation. A means of providing fixation immediately on implantation is also required.
Implantable dental devices for implantation into the root canal space of a tooth are known as dental posts. Dental posts are rod-like devices that are positioned and fixated within the root canal space of a damaged tooth to reinforce compromised structural integrity of the tooth. Dental posts can also comprise an abutment to provide means from attachment of a dental prosthesis. The choice of dental post to be used depends on a number of issues including the type of damaged tooth. In this regard, different tooth types have different root numbers and configurations requiring different post configurations. In addition the type of surgical procedure, for example root canal surgery or tooth reconstruction following trauma, influences the choice of dental post. Similar to implantable fixation devices for implantation into alveolar bone, the majority of known dental posts are cylindrical devices formed from titanium or a titanium-based alloy, which can be inserted into a pre-prepared drilled hole in a root canal of a tooth.
Problems associated with known dental implants include inadequate instant and long-term fixation, implant rejection, poor aesthetics and a need for complex and repetitive surgical procedures.
Known implants are produced in a range of sizes, and dimensions. The choice of implant is dependent on a number of issues including the desired location, avoidance of integral tissue structures and the quality of the bone or dental tissue in which the implant is to be implanted. Incorrect choice of implant dimensions, including geometry and thread size in screw-type implants can accentuate insufficient or excessive mechanical loading which can lead to implant loosening and failure. Implant failure necessitates implant removal and repetitive surgical procedures.
It is imperative that a good seal is formed between an implant and an implantation cavity. A seal can be produced by use of cements or fillers or by tailoring geometry of an implant, for example providing a tapered head to a screw-type implant. However, despite use of these techniques, it can be difficult for a thorough seal to be achieved. A substandard seal can allow entry of bacteria and lead to infection of underlying bone or dental tissue. If infection occurs, implant removal may be necessary. A substandard seal may also allow entry into the implantation cavity of soft tissue, which can lead to fibrous tissue formation. This can contribute to implant failure, for example by preventing osseointegration.
Often a desired site of implantation will comprise poor quality bone or dental tissue and it is well known that fixation of an implant in poor quality tissue is likely to be poor. There are also situations where the bone or dental tissue present is of good quality, but limited in quantity. Fixation is likely to be poor if the quantity of bone or dental tissue present is limited. Generally, if initial fixation of an implant is insufficient, as tested by standard dental techniques such as load application or acoustic tests, a temporary cap is placed over the implant for as long as is necessary to achieve adequate integration and fixation of the implant, for example by osseointegration, before any further work can be carried out. This can take as long as six months.
Moreover, although pure titanium implants have good corrosion resistance and strength characteristics, their use can cause undesirable greying of gums and crown materials. Implants formed of titanium alloys can also cause problems due to the galvanic difference of the metals which can lead to corrosion, implant loosening and failure.
Known dental implants require the use of an invasive surgical technique. Traditionally, dental practitioners drill a hole in the jaw bone or root canal of a tooth, the size of which is determined by the chosen implant to be inserted or screwed in. This invasive procedure can cause damage to surrounding tissue which can affect fixation of the implant.
Dental implantation techniques and implant devices have been developed which seek to address the problems discussed above. One such technique is the use of liquefied cements to achieve enhanced dental fixation. The use of a liquefied cement, however, necessitates a multi-step, multi-component implantation procedure involving drilling of a hole of the required size to make room for the dental implant, cleaning of the whole, positioning of a dental implant in the hole and subsequent fixation in place by a dental cement.
Shape memory alloys such as Ni—Ti alloys have been suggested as enhanced fixation materials for forming dental implants. These materials, upon induction with heat, are able to expand to a pre-configured shape enabling enhanced implant fixation. U.S. Pat. No. 5,108,289 discloses a dental endosseous implant comprising a thermal shape memory material, specifically a Cu—Zn—Al or Ti—Ni alloy. The dental implant, once inserted in the bone and upon induction with heat, changes shape to seek enhanced fixation within the alveolar bone. U.S. Pat. No. 5,951,288 discloses an implant containing three separate components, a root portion, a neck portion and an abutment. The root portion consists of three legs comprising a shape memory alloys (Ni—Ti, Ti—Pd or Ti—Pd—Co alloy) which upon induction with heat separate to provide fixation of the implant.
The use of shape memory alloys seeks to provide enhanced fixation and enhanced integration by exertion of a compressive force on the surrounding bone. Compressive forces have been shown to stimulate biological processes including osseointegration (‘Increased calcification of the growth plate cartilage as a result of compressive force in-vitro’ Nulend et al. Arthritis & Rheumatism, Vol. 29 (8), 1002-1009, 1986; ‘Inhibition of osteoclastic bone resorption by mechanical stimulation In Vitro’ Nulend et al. Arthritis & Rheumatism, Vol. 33 (1), 66-72, 1999).
However, shape memory alloys are expensive. Furthermore, the use of shape memory alloys in dental implants can lead to problems associated with non-degradability and a lack of biocompatibility. For example, alloys containing nickel can initiate an allergic response in some people.
Shape memory polymers are known and have been described in U.S. Pat. No. 4,950,258 and U.S. Pat. No. 6,281,262 for use in medical devices such as tissue suture devices, blood vessel expanders, tendon and bone fixation devices.
Shape memory polymers have had very limited use in the dentistry field. U.S. Pat. No. 6,299,448 discloses a multi-component device for implanting into gums or alveolar bone which can be used to provide support for subsequent implants. Implantation of this device requires a multi-step, multi-component process. The device comprises a stent-like anchor formed from a coil, helix, mesh or tube of a shape memory material. The stent-like anchor, which defines an internal cavity, is covered by a porous sleeve. The stent-like anchor and the sleeve together form a stent assembly which is implanted to line an alveolar cavity. Once the stent assembly has been implanted, in order to activate the shape memory material and to provide the implanted device with internal structure, it is necessary for a polymerisable material to be injected into the central cavity defined by the stent assembly. Exothermic polymerisation of the polymerisable material generates heat sufficient to activate the shape memory material.